At the present time there is no known good solution to the general problem of energy storage. A good solution is one that needs to have as many of the following characteristics as possible:
1) High energy density per volume and per mass
2) Low cost of manufacture
3) Unlimited number of charge/discharge cycles
4) Capable of fast charge/discharge
5) Insensitivity to temperature
6) Manufactured from non-toxic, readily available materials
Li-ion batteries, and most chemical batteries, meet criteria 1) and 2) but fall short on all the others. State-of-the-art Electrochemical Double Layer Capacitors (EDLCs) fall short on 1) and 2) but satisfy the remaining criteria adequately.
Li-Ion batteries have an energy density of around 400 Watt-hours/liter (W-Hr/l) while EDLCs typically have an energy density of around 10 W-Hr/l. However, UltraCaps can be cycled millions if not tens of millions of times, whereas Li-Ion batteries can be cycled, generally, at most 1000 times before wearing out. Further, Li-Ion batteries are low cost as well, usually around $1 per W-Hr, whereas EDLCs currently cost about $13 per W-Hr.
FIG. 1A shows the relationship between energy density, dielectric constant K and electric field strength E for a given capacitor. Note that while the energy density is linear in K it goes as the square of E. To maximize energy density it is necessary to optimize the KE2 product. Energy density (energy per volume) in a conventional parallel plate capacitor is given by:½Kε0E2,with units of Joule/m3, and where E (V/m) is the electric field, K is the dielectric constant (K=1 for vacuum, K=3.9 for pure SiO2), and ε0 (Farad/m) is the permittivity of free space. Note that the energy density goes as the square of e and is only linear in K.
FIG. 1B shows that as the voltage across the plates of the capacitor is increased electric field strength E and energy density monotonically rise—along with leakage currents between the plates. Leakage currents allow the capacitor to discharge itself, leading to a reduction in ‘retention time’, that is, the amount of time it takes for a capacitor to lose a certain fraction of its initial charge. As a practical matter leakage currents place an upper limit on the KE2 product. For a given capacitor, the maximum applied voltage (and therefore the E-field and the energy density) is limited by leakage currents between the plates. Increased leakage currents lead to reduced retention time (e.g., time to lose 10% of stored energy). Thus there is a tradeoff between energy density and retention time. The higher the former (i.e., the energy density) the lower the latter (i.e., retention time). The magnitude of leakage will be dependent on the choice of materials used to construct the capacitor (including the electrodes and the dielectric fill), as well as its structural architecture.
As the importance of and demand for energy storage continues to increase, there is a need for an energy storage technology that has the characteristics described above.